1. Field of the Invention
The present invention relates generally to a method and apparatus for allocating channels in a wireless communication system, and in particular, to a method and apparatus for allocating channels in an Orthogonal Frequency Division Multiple Access (OFDMA) system.
2. Description of the Related Art
Wireless communication systems have been developed to make up for shortcomings of wired communication systems in which subscriber stations cannot access fixed wired networks. With the progress of communication technology, the wireless communication systems have evolved into mobile communication systems. A cellular system is a typical mobile communication system. The cellular system refers to a system for connecting a base station to a subscriber station through a wireless channel to a wired network. A cellular mobile communication system using a Code Division Multiple Access (CDMA) scheme (hereinafter referred to as a “CDMA cellular mobile communication system”) is a typical cellular system.
Although the cellular systems were originally developed to provide voice communication services, cellular systems capable of providing various data services have recently appeared. In addition, as the amount of data required by each cellular system user increases, so does the demand to transmit this data at higher speed. Therefore, ongoing research into a CDMA cellular system capable of meeting these demands is being made.
Also, research into an OFDMA system (different from the CDMA system) is being made to provide users with the desired data at high speed. In addition, commercialization of the OFDMA system is now being discussed.
A detailed description will now be made of the OFDMA system. The OFDMA scheme identifies users using orthogonal frequencies in transmitting data to the users. The OFDMA scheme is superior to the CDMA scheme in terms of a data rate. However, the use of the OFDMA scheme inevitably increases a frequency reuse factor.
With reference to FIG. 1, a description will be now made of frequency reuse. FIG. 1 is a conceptual diagram for a description of frequency reuse in an OFDMA cellular system. A frequency reuse factor is 3, by way of example. Available orthogonal frequencies are represented by carrier indexes. A “carrier index” refers to a set of orthogonal frequencies with which users can be identified. Not all of the orthogonal frequencies can be used in a particular base station because orthogonal frequencies used in a first base station cannot be used in a neighbor region of the base station, as doing so can disable data communication due to interference between two similar frequencies.
In order to solve this problem, it is necessary to allocate different frequencies to base stations so that the same frequency should not be used in a neighbor region of a particular base station. As illustrated in FIG. 1, assuming that reference numeral 100 denotes a base station located in the center, all of neighbor base stations 110, 120, 130, 140, 150 and 160 have different carrier indexes. That is, the respective base stations use frequencies in such a manner so that no interference can occur between the frequencies. More specifically, n1 represents a set of carrier indexes for ⅓ of all orthogonal frequencies. Base stations using orthogonal frequencies with the carrier indexes included in the carrier index set n1 are denoted by reference numerals 110, 130 and 150. Similarly, n2 represents a set of carrier indexes for another ⅓ of all orthogonal frequencies. A base station using orthogonal frequencies with the carrier indexes included in the carrier index set n2 corresponds to a base station 100 located in the center. Finally, n3 represents a set of carrier indexes for another ⅓ of all orthogonal frequencies. Base stations using orthogonal frequencies corresponding to the carrier indexes included in the carrier index set n3 are denoted by reference numerals 120, 140 and 160. For such a hexagonal cellular system, a frequency reuse factor is 3. This means that every 3 base stations can use the same frequency. In other words, it means that frequencies available in one base station become ⅓ of all available frequencies. It is assumed that each base station in FIG. 1 has an ideal hexagonal cell. However, in an actual cellular system, each base station cannot have an ideal hexagonal cell, thereby necessitating an increase in number of neighbor base stations. Therefore, the frequency reuse factor may be larger than 3 in actual use.
The increase in frequency reuse factor results in a reduction of frequency resources available in an OFDMA cellular system, thus decreasing user capacity.
Currently, however, a system capable of using a frequency reuse factor of 1 or a system capable of using a frequency reuse factor approximating 1 is being considered for an OFDMA cellular system. In this case, it is not possible to support handover from a particular base station to another base station for the following reason. In the case where a frequency reuse factor of 1 or a frequency reuse factor approximating 1 is used, if a subscriber station moves from a particular base station to another base station, the subscriber station should use both a frequency resource used in the old base station and a frequency resource allocated in the new base station, causing a considerable increase in interference between the frequency resources.
More specifically, a subscriber station in handover operation is allocated a particular orthogonal frequency resource for communication with its old base station (hereinafter referred to as a “serving base station”) and is performing communication with the serving base station. Herein, the term “handover situation” refers to a situation in which a subscriber station is moving from a serving base station to a new base station (hereinafter referred to as a “target base station”). In this case, the target base station might have already allocated the same frequency resource to another subscriber station. At this point, the subscriber station in handover operation and another subscriber station located in the target base station use the same frequency resource, causing a considerable interference. In this case, both of the two subscriber stations cannot perform communication.